Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user's end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user's end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines.
Generally, governmental regulations specify the telephone interface requirements and specifications for a variety of parameters including pulse dialing transitions, spark quenching, AC termination, DC termination, ringer impedance, ringer threshold, etc. For example, Federal Communications Commission (FCC) Part 68 governs the interface requirements for telephones in the United States. However, the interface requirements world wide are not standardized, and thus, in countries other than the United States the applicable standards may include the CTR21, TBR21, NET4, JATE, and various country specific PTT specifications. Because the interface requirements are not standardized from country to country, often different DAA circuitry is required for use in each country in order to comply with the appropriate standard. The requirement for different DAA circuitry, however, limits the use of one phone line interface in a variety of countries. Thus, for example, a modem in a laptop computer configured for interfacing with a phone line in one country may not necessarily operate properly in another country. Further, the requirement for different DAA circuitry in various countries hinders the design of a single integrated cost effective DAA solution for use world wide.
As mentioned above, the telephone interface requirements generally include specifications for the pulse dialing (also called decadic dialing) transitions and spark quenching presented to the telephone line. In general, pulse dialing comprises a repetitive series of on-hook and off-hook transitions. FIG. 1 shows the standard two-wire public network lines, the TIP line 8 and the RING line 6. The TIP line and the RING line may be conventionally connected to a diode bridge 11. The diode bridge presents the proper polarity line signal to the hookswitch circuit 12 independent of the TIP and RING polarity. The hookswitch circuit 12 operates as a switch to “seize” or “collapse” the TIP and RING phone lines to allow the maximum loop current (Iloop) that is available from the phone line to flow. In an on-hook condition (i.e. the user is not transmitting data to or from the phone line), the hookswitch circuit 12 may be switched open. In an off-hook condition, the hookswitch circuit 12 may be switched closed to allow a loop current flow ILOOP. The remaining DAA circuitry is shown as block 10. The phone company exchange is connected to the other side of the TIP and RING lines and may be characterized as a voltage source 16, an inductor 14 have an inductance L and a resistor 13. As the hookswitch opens and closes, the loop current flow ILOOP will change and the voltage across the inductor 14 will change.
FIG. 1A illustrates the voltage across the inductor as the state of the hookswitch changes. FIG. 1A shows an exemplary series of hookswitch transitions. Such a series of transitions may be seen at a period of 100 msec, for example, during pulse dialing. It will be noted that the voltage across the inductor spikes as shown by dashed lines 22 during transitions from an off-hook condition to an on-hook condition (i.e. the loop current ILOOP transitions from a steady state off-hook value to zero). The voltage spike 22 results from the inductor V-I relationship V=L(di/dt) that results since the phone company exchange is characterized as an inductive source. If the loop current suddenly drops, large voltage spike will occur across the effective inductance of the phone company exchange, and thus, across the TIP and RING lines. The maximum inductor voltage is specified in various countries and is sometimes referred to as the “spark quenching” specification. For example, in Australia (one of the more demanding specifications), the line inductance is specified as 4H and voltage across such an inductance may not exceed 230V. These voltage spikes may also result in undesirable voltage sparks across the hookswitch.
In addition to the spark quenching specifications, for pulse dialing some countries have specifications which require the transition from off-hook to on-hook to occur slowly. Thus, another country dependent specification exists which may require the hookswitch transition to be controlled.
One prior art approach to limit the instantaneous current change which occurs when the hookswitch is changed from off-hook to on-hook is shown in FIG. 1B. As shown in FIG. 1B, a resistor 32 and capacitor 30 are provided around the hookswitch circuit 12. The purpose of the resistor 32 and capacitor 30 is to provide a current path around the hookswitch when the hookswitch is opened. The RC effect of the resistor 32 and capacitor 30 is to slow the current change from the steady state off-hook value to the on-hook zero value. Thus, the di/dt term will be decreased and the maximum voltage seen across the inductor will drop since the spike 22 will decrease. Other prior art approaches may include the use of a voltage clamp (such as an MOV device) placed across the TIP and RING lines. The use of these additional discrete external devices add to the DAA system costs and complexity.
It is desirable, therefore, to provide a DAA circuitry that may be suitable for use in many or all countries without the need for use of additional external discrete devices to satisfy off-hook and on-hook transition standards.
Further, it is also desirable that the DAA circuitry act as an isolation barrier since an electrical isolation barrier must exist in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. For example, in order to achieve regulatory compliance in the United States with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
Thus, there exists a need for reliable, accurate and inexpensive DAA circuitry for satisfying the hookswitch transition standards for multiple country phone line standards and a DAA circuitry which also provides the necessary electrical isolation barrier.